Field of the Invention:
The present disclosure relates, in general, to skins for an aircraft, and in particular, to skins for a tiltrotor aircraft wing.
Description of Related Art:
Modern aircraft are manufactured from a wide variety of materials, including steel, aluminum, and a wide variety of composite materials. Most structural components are made from strong, rigid materials. However, in order to conserve weight, the structural components are often made from a thin layer of metal or composite that includes reinforcement strips of material reinforced with stringers.
Tiltrotor aircraft have complicated proprotor assemblies located at opposing wing tips that operate between a helicopter mode to take off, hover, fly, and land like a conventional helicopter; and an airplane mode. The proprotor assemblies are oriented vertically for a helicopter mode and horizontally for airplane mode. Because the tiltrotor aircraft must operate in both helicopter mode and airplane mode, and operate while transitioning between the two, the wing structure must support the weight of the proprotor assemblies, withstand the forces generated from the proprotor assemblies in a variety of modes, and provide a lifting force sufficient to lift the weight of the aircraft.
FIG. 1 is a partial view of an exemplary prior art tiltrotor wing 10 including a torque box structure 30. The torque box structure 30 includes skins 20, forward spar 32, and aft spar 34. The skins 20 includes stringers 12 extending generally parallel to the longitudinal axis of the wing 10. The upper skin 22 requires five stringers 12 and the lower skin 24 requires four stringers 12. The stringers 12 provide stiffness and support to the skin 20 and are each an I-beam shaped stiffener as shown in FIG. 2 connected to the interior surface 20a of the skin 20. The stiffeners 12 are made from a composite material and extend the depth of the skin 20 assembly into the interior of the wing 10 thereby reducing the space available for fuel and other internal systems.
The skin 20 is constructed of many of layers or “plies” of composite materials including hundreds of reinforcement strips 28 or “postage stamps” made of various types, sizes, orientations, and thicknesses of materials. The reinforcement strips 28 are made of graduated sizes of postage stamp stamps that have been compacted together as shown in FIG. 2. The reinforcement strips 28 are located below the stringer 12: (1) to provide support for the skin 20 against catastrophic buckling; (2) to maintain shape and contour of the skin 20; (3) to provide stiffness at the stringer load points; and (4) to distribute pressure into the skin.
During manufacture of the skin 20 each of the reinforcement strips 28 is cut, labeled, and positioned in a mold, which is an extremely time-consuming and laborious process. When the size and shape of a reinforcement strip 28 is used repeatedly, a problem results in that the reinforcement strips 28 are pre-cut and stored in a controlled atmosphere environment and must be identified and thawed by a user each time a reinforcement strip is needed for a composite.
The stringers 12 are connected to the torque box structure 30 using rivets or other suitable means. The torque box structure 30 further includes lower supports 36, and upper supports 38. The lower and upper supports 36, 38 provide horizontal structural strength to the forward and aft spars 32, 34 and to the respective upper and lower skins 22, 24. The lower and upper supports 36, 38 are stiffening elements to keep the rib from buckling and act as a doubler around an access hole through the rib. The lower and upper supports 36, 38 are individual manufactured composite parts that are mechanically fastened during the rib install, which increases the part count and time for assembly of the overall wing structure. As shown in FIG. 1, the torque box structure 30 includes multiple internal supports that reduces the space available for fuel and other internal systems.
The assembly of the torque box structure 30 is very complex, often with very tight tolerances required. The installation of the fasteners to the skins 20 and other structural components is also difficult because there is limited access to small interior spaces and complicated sealing requirements. Moreover, a large number of fasteners is required for each wing 10, which can cause the structures to warp and dimensional growth during assembly. Once the structural members are assembled, over a hundred foam details are positioned between the structural members in the fuel bays to provide a smooth, ramped surface for the fuel components housed therein. The assembly of the torque box 30 is time consuming and extremely labor intensive at each of the various stages of manufacture (manufacture of the composites, sub-assembly, installation stages).
The wing structure in FIGS. 1 and 2 is a cross-sectional view of a prior art tiltrotor swept, dihedral wing that concentrates loads at the outboard ends and inboard ends adjacent to the fuselage; which requires structural reinforcement in those areas to withstand twisting and torsional forces during the various flight modes. The front spar requires three spars and the back requires five spars along with tip spars to provide sufficient structural strength for the swept, dihedral wing.
Accordingly, the need has arisen for an improved wing structure, assembled components, and methods for manufacture thereof for use on a tiltrotor aircraft that addresses one or more of the foregoing issues.